1. Field of the Invention
This invention relates to a heat-insulating piston consisting of a composite structure containing a ceramic member and a heat-insulating member.
2. Description of the Prior Art
Conventionally, a heat-insulting piston is disclosed in Japanese Patent Laid-Open No. 302164/1988. This heat-insulating piston will be explained with reference to FIG. 6 of the accompanying drawings.
This heat-insulating piston comprises a piston head portion 41 having a fitting boss portion 44 at its center and made of a material having a thermal expansion coefficient substantially equal to that of a ceramic material, and a metallic piston skirt portion 42 having a center fitting hole 52 to which the fitting boss portion 44 is fitted at its center. The fitting boss portion 44 of the piston head portion 41 and the center fitting hole 52 of the piston skirt portion 42 are fixed to each other by metal flow of a metallic ring 51. A heat-insulating buffer material 48 as a heat-insulating gasket is interposed under a push state at the center contact portion between the piston head portion 41 and the piston skirt portion 42. A heat-insulating air layer 49 is defined between the piston head portion 41 and the piston skirt portion 42.
Furthermore, a ceramic thin sheet 45, which is formed to an extremely small thickness in order to reduce thermal capacity, is disposed on the piston head portion 41 through a heat-insulating material 43 in such a manner as to face a combustion chamber. A ceramic ring 46 made of the same material as the ceramic thin sheet 45 is fitted to the outer peripheral portion of the latter, and these ceramic thin sheet 45 and ceramic ring 46 are bonded at the contact portion by CVD (Chemical Vapor Deposition) as described in Japanese Patent Laid-Open No. 108171/1989 (U.S. Pat. No. 4,848,291), for example.
A step portion 56 is formed on the inner peripheral surface of the ceramic ring 46 and the outer peripheral portion of the piston head-portion 41 fits to the ceramic ring 46 so as to come into contact with the step portion 56 of the ceramic ring 46. A heat-insulating material 43 is sealed into the space defined by the ceramic thin sheet 45, the ceramic ring 46 and the piston head portion 41. This heat-insulating material 43 is made of a material such as potassium titanate whiskers, zirconia fibers. When the piston head portion 41 is fitted to the piston skirt portion 42 under the push state, the outer peripheral portion of the piston head portion 41 is pushed to the step portion 56 of the ceramic ring 46 and the ceramic ring 46 is pushed to the peripheral portion of the piston skirt portion 42. A carbon seal 47 as a gasket is interposed in order to provide sealing between the ceramic ring 46 and the piston skirt portion 42.
In a heat-insulating engine member using a ceramic material as a heat-insulating or heat-resistant material such as a piston, it is extremely difficult to obtain sufficient heat-insulating characteristics. The ceramic material is kept under the state where it is exposed to high temperatures on the combustion chamber side and there exist the problems, therefore, that the ceramic material receives a thermal shock and its strength is not sufficient. If the thickness of the ceramic material on the wall surface is increased for the purpose of heat insulation, a thermal capacity becomes greater and there occur the problems that intake air receives a greater quantity of heat from the combustion chamber and is heated to high temperatures during an intake stroke, its heat affects the intake air, suction efficiency drops and air cannot be sucked, whereas the heat-insulating property must be improved in an expansion stroke, on the contrary.
To solve these problems, the structure of the heat-insulating piston disclosed in Japanese Patent Laid-Open No. 302164/1988 is as described above in order to obtain extremely high heat-insulating property, to minimize the thermal capacity of the surface portion of the piston head which is exposed to the combustion gas and reaches high temperature, to improve intake efficiency and cycle efficiency, to eliminate the occurrence of the problems of strength even when a thermal shock is applied, to improve heat resistance, corrosion resistance and deformation resistance, to obtain a stable fitting state and to receive under a preferred state the pressure which acts on the piston head at the time of explosion. Further, it improves the seal function between the piston head and the piston skirt.
In the heat-insulating piston described above, the heat-insulating material disposed between the head base portion and the ceramic thin sheet disposed on the combustion chamber side is composed of whiskers or fibers of mullite, alumina, possium titanate, zirconia, or the like, and the ceramic thin sheet and the ceramic ring are made of a ceramic material such as silicon nitride. Therefore, since the materials are different between the heat-insulating material, the ceramic thin sheet around the former and the ceramic ring and their thermal expansion coefficients are therefore different, the difference of thermal expansion occurs between the different materials after the ceramic thin sheet and the ceramic ring are bonded mutually and a gap develops between the ceramic thin sheet as the surface of the piston head and the heat-insulating material. This is structurally disadvantageous to the explosion force at the time of combustion and results in the breakdown of the ceramic thin sheet.
Moreover, if the bond portion between the ceramic thin sheet and the ceramic ring is bonded by chemical vapor deposition or coating, the bond portion does not have the strength sufficient to keep the bonded state against the explosion force at the time of combustion, so that the bond portion between the ceramic thin sheet and the ceramic ring peels or cracks develops.